1. Field of the Invention
The invention relates to a drilling grab for pile foundations and well boreholes.
2. Description of the Prior Art
Drilling grabs of this type serve in particular for producing drill holes in the ground for concrete piles. They consist of a grab body, which supports circular grab spades at its end, which spades are able to tilt from an open position into a closed position about pins located in the grab body.
In these known drilling grabs, the opening and closing of the spades takes place by a mechanism in the form of a toggle lever actuated externally, for example, by a cable. The grab spades are kept open by a locking mechanism; and when the drilling grab is lowered, the open spades penetrate the earth under the action of the weight, whereupon they are closed by the actuation of the opening mechanism and withdrawn from the drill hole with the earth carried therebetween.
In another known drilling grab, the grab spades are opened by spring force and closed against the spring force by the cable of a winch. In this case, provided between the grab body and the opening mechanism of the grab spades actuated by a tie-rod attached to the winch cable is a spring force produced by one or more springs, which force is less than the weight of the grab body, but is so great that it opens the grab blades during the descent of the grab with additional acceleration of the winch cable and winch drum. When lowering the drilling grab by means of the winch cable attached to the tie-rod, the spades are kept closed against the spring force by the weight of the grab body. At the drilling point, the winch cable and winch drum are released so that the drilling grab falls freely. In this case, the grab spades are opened by the spring force coming into operation so that they may penetrate the earth. When pulling on the winch cable with the grab body in the inoperative position, the grab spades are first closed against the action of the spring force, whereupon the grab body with the closed spades is raised by the winch cable. A drilling grab of this type is described in German Pat. Nos. 1,058,953 and 1,280,776.
In all known drilling grabs, in the open position of the spades, the backs of the spades are supported on the lower edge of the grab body in order to transmit to the grab body the forces acting on the blades during the penetration of the spades into the ground. To achieve a varying angle of opening of the grab spades, it is already known to detachably connect intermediate parts serving as abutments to the front edge of the grab body and to the backs of these spades. In drilling grabs, it is also known to provide a radially projecting support on the periphery of the grab body for the spades adapted in their dimensions to the drilling grab housing, which support forms an abutment for the backs of the grab spades located in the open position slightly overlapping the periphery of the grab body. This abutment surrounds part of the grab spades and is attached to the grab body by screws. With this known abutment the forces occurring in the radial and axial direction of the grab body, produced by the spades, must be absorbed by the securing screws of the abutment, which must lead to rapid wear of the securing screws and thus also of the abutment, when they have been in operation for a long time.
In known drilling grabs, the diameter of the grab body with its spades is designed for a certain diameter of the drill hole. In the production of drill holes, in particular for pile foundations, it is nevertheless necessary to produce drill holes with considerably varying diameters, in order that the foundation piles inserted in the drill holes correspond to the necessary loads. Drill holes are generally required to have diameters of 850, 900, 1000, 1100, 1200, and 1250 mm. In order to eliminate the storage of a corresponding number of different drilling grabs, it is therefore necessary to be able to use a drilling grab housing of the smallest dimension by exchanging spaces of corresponding dimensions for the production of various drill holes. Different spades have already been associated with one and the same grab body, in which the inclination of the shovel blades to the back of the spade is different, so that when the grab spades are open, different distances corresponding to the desired diameter of the drill hole occur between centers. When using spades with larger distances between centers, the drawback exists that spades with shovel blades directed obliquely outward have great difficulty in penetrating solid ground. Furthermore, the bearings and pivot points of the spades are subject to such considerable loads, which result in rapid wear, that the use of such exchangeable blades is prohibited. In order to avoid these drawbacks, the grab body is constructed of two halves divided at right angles to its axis, which halves may be connected to each other by flanges and in which the lower halves of the grab body are adapted to the desired diameter of the drill hole. In this case, it is naturally necessary to have available a number of lower halves of the grab body corresponding to the number of different diameters of the drill holes. This naturally involves considerable cost and additionally considerable expenditure of energy during the necessary exchange of the lower halves of the grab body and the spades, especially since the exchangeable lower halves of the grab body have considerable weight.